1. Field of the Invention
The present invention relates to imagers and, more particularly, to an imager with improved sensitivity.
2. Description of the Related Art
A black and white imager is a device that utilizes a photodiode to convert light into photo-generated electrons, and a transistor circuit to collect and determine the number of photo-generated electrons that were generated during a time interval commonly known as an integration period.
A color imager is a device that converts light, which includes a number of different wavelengths, into photo-generated electrons, and then separately collects the photo-generated electrons that were generated from the different wavelengths of light. For example, a color imager can separately collect the red electrons that were formed from red light, the green electrons that were formed from green light, and the blue electrons that were formed from blue light. The color imagers in digital cameras utilize millions of color cells that capture images based on the amount of red, green, and blue light that strikes the cells.
One approach to implementing a color imager is to utilize a first imaging cell with a red filter that lies over a first photodiode, a second imaging cell with a green filter that lies over a second photodiode, and a third imaging cell with a blue filter that lies over a third photodiode. The red filter of the first imaging cell only passes red light which, in turn, allows the first photodiode to produce and collect only red photo-generated electrons.
Similarly, the green filter of the second imaging cell only passes green light which allows the second photodiode to produce and collect only green photo-generated electrons, and the blue filter of the third imaging cell only passes blue light which allows the third photodiode to produce and collect only blue photo-generated electrons. One drawback with this approach is that three photodiodes are required which, in turn, requires a substantial amount of silicon real estate.
Another approach to implementing a color imager that addresses the size problems of the three-cell approach is a vertical color imager. A vertical color imager is an imager that separately collects the photo-generated electrons that represent the different colors, such as red, green, and blue.
A vertical color imager can be implemented with a number of vertical color imaging cells that each include, for example, six regions of alternating p-type and n-type conductivities that form five vertically-aligned pn junctions. The first pn junction, which is furthest from the top surface of the top region, forms a red pn junction, while the third pn junction forms a green pn junction and the fifth pn junction, which is closest to the top surface of the top region, forms a blue pn junction.
The red pn junction has a depth from the top surface of the top region that is approximately equal to the absorption depth of red light in silicon. In addition, the green pn junction has a depth from the top surface that is approximately equal to the absorption depth of green light, and the blue pn junction has a depth from the top surface that is approximately equal to the absorption depth of blue light.
Electrical connections to the five lower regions can be made by forming sinker regions that extend from the top surface down to the regions. For example, an electrical connection to the bottom n region can be made with an n+ sinker region that vertically extends from the top surface to the bottom n region. The electrical connections to the regions are typically formed in a square or rectangular fashion, leaving a center area free from obstruction.
In addition, a transistor circuit, which determines the number of photo-generated electrons that were generated and collected during an integration period, can also be formed around the center area on the top surface of the top region. The transistors utilize a relatively small amount of space with respect to the amount of area utilized by the photodiode. Thus, one of the advantages of a vertical color imaging cell is that since the cell collects multiple colors, a vertical color imaging cell is only slightly larger than a black and white imaging cell.
One example of a vertical color imaging cell is taught by U.S. Patent Application Publication US 2002/0058353 A1 published on May 16, 2002. FIG. 1 shows a combined cross-sectional and schematic diagram that illustrates a prior art vertical color imaging cell 100. Cell 100 is substantially the same as the cell shown in FIG. 2A of the '353 published application.
As shown in FIG. 1, imaging cell 100 includes a red p− region 110, a red n+ region 112 that contacts p− region 110, and a red depletion region 114 that is formed at the junction between regions 110 and 112. Imaging cell 100 also includes a green p− region 120 that contacts n+ region 112, a green n+ region 122 that contacts p− region 120, and a green depletion region 124 that is formed at the junction between regions 120 and 122. In addition, imaging cell 100 further includes a blue p− region 130 that contacts n+ region 122, a blue n+ region 132 that contacts p− region 130, and a blue depletion region 134 that is formed at the junction between regions 130 and 132.
Further, imaging cell 100 includes first, second, and third reset transistors 150, 152, and 154 which are connected to n+ regions 112, 122, and 132, respectively, while p− regions 110, 120, and 130 are connected to ground. Prior to collecting photo information, reset transistors 150, 152, and 154 are pulsed on which, in turn, places a positive potential on n+ regions 112, 122, and 132.
The positive potential reverse biases the pn junction of regions 110 and 112, thereby increasing the width of red depletion region 114, and the pn junction of regions 120 and 122, thereby increasing the width of green depletion region 124. The positive potential also reverse biases the pn junction of regions 130 and 132, thereby increasing the width of blue depletion region 134.
Once the positive potentials have been placed on n+ regions 112, 122, and 132, light energy, in the form of photons, is collected by the red, green, and blue photodiodes. The red photons are absorbed by the red photodiode which, in turn, forms a number of red electron-hole pairs, while the green photons are absorbed by the green photodiode which, in turn, forms a number of green electron-hole pairs. Similarly, the blue photons are absorbed by the blue photodiode which, in turn, forms a number of blue electron-hole pairs.
The red electrons from the electron-hole pairs that are formed in depletion region 114 move under the influence of the electric field towards n+ region 112. On the other hand, the holes formed in depletion region 114 move under the influence of the electric field towards p− region 110.
In addition, the electrons, which are from the electron-hole pairs that are formed in p− region 110 within a diffusion length of depletion region 114, diffuse to depletion region 114 and are swept to n+ region 112 under the influence of the electric field. Further, the electrons that are formed in n+ region 112 remain in n+ region 112. Each additional electron collected by n+ region 112 reduces the positive potential that was placed on n+ region 112 by reset transistor 150.
Similarly, the green electrons from the electron-hole pairs that are formed in depletion region 124 move under the influence of the electric field towards n+ region 122. On the other hand, the holes formed in depletion region 124 move under the influence of the electric field towards p− region 120.
In addition, the electrons, which are from the electron-hole pairs that are formed in p− region 120 within a diffusion length of depletion region 124, diffuse to depletion region 124 and are swept to n+ region 122 under the influence of the electric field. Further, the electrons that are formed in n+ region 122 remain in n+ region 122. Each additional electron collected by n+ region 122 reduces the positive potential that was placed on n+ region 122 by reset transistor 152.
As with the red and green electrons, the blue electrons from the electron-hole pairs that are formed in depletion region 134 move under the influence of the electric field towards n+ region 132. On the other hand, the holes formed in depletion region 134 move under the influence of the electric field towards p− region 130.
In addition, the electrons, which are from the electron-hole pairs that are formed in p− region 130 within a diffusion length of depletion region 134, diffuse to depletion region 134 and are swept to n+ region 132 under the influence of the electric field. Further, the electrons that are formed in n+ region 132 remain in n+ region 132. Each additional electron collected by n+ region 132 reduces the positive potential that was placed on n+ region 132 by reset transistor 154.
After the red, green, and blue photodiodes have collected light energy for an integration period, sense circuitry associated with the photodiodes detects the change in potential on n+ regions 112, 122, and 132. Specifically, in addition to a reset transistor, each photodiode also has an associated source follower transistor SF and a row select transistor RS.
The change in potential on an n+ region is present on the gate of the associated source follower transistor SF, while the source of the source follower transistor SF is one diode drop below the potential. Thus, when the gate of the row select transistor RS is pulsed, an output potential equal to the photodiode potential less a diode drop is output to a sense cell to determine the output potential. Once the change in positive potential has been determined, the photodiodes are reset and the process is repeated.
One problem with imaging cell 100 is that, in addition to moving vertically to a contact point located on the top surface, many of the red, green, and blue electrons must also move laterally to be collected. The electrons, however, have relatively-short, recombination lifetimes which, in turn, limits the distances the electrons can travel.
Another problem with vertical color imaging cell 100 is that the electrons from one n region, such as the green n+ region 122, can be collected by another n region, such as the red n+ region 112. This cross talk, where electrons from one n region are collected by another n region, reduces the sensitivity of the cell.